Macbook 12" A1534 built-in LCD becomes black, but external display still works

I have a macbook 12" A1534. The built-in LCD display works abnormally. At the beginning, the screen could be black randomly when the machine started up. Two weeks ago, it becomes totally dead. I am sure the system is starting because I can see the backlight of the keyboard is on. Today, I got type-c to HDMI adaptor and connected the laptop with my TV using HDMI cable. On my TV, I can see the system started normally, but the built-in LCD display is still black. I already attempted to reset SMC and NVRAM, but I am not sure if I did properly. Anyway, the screen is still back. Does anyone have any idea how to fix this problem. Obviously, the machine is out of the warranty. If I ask for the help from Apple, how much will it cost me to fix this problem? Thanks for any suggestions.